Head-up displays (HUD) are systems which project images onto a viewing surface at a position which allows the viewer to maintain a posture in which the gaze is directed forward rather than downward to a display or instrument panel. Head-up displays are used in various environments such as motor vehicles, aircraft, helmets and other situations in which it is important that the viewer not divert his gaze.
Although head-up displays are useful in such environments, the images can sometimes be difficult to discern in bright ambient lighting conditions. Therefore, there is a need in the art for displays which can be easily viewed in bright light. Previously, attempts have been made to harvest ambient light for head-up displays. An example of such a head-up display is found in WO 95/13557 which uses an ambient light source in a display which projects images onto a windshield. Attempts have also been made to combine ambient light with another light source as in U.S. Pat. No. 4,997,263 and U.S. Pat. No. 7,430,349. Although ambient light and another light source are used, the former patent has low ambient light collection efficiency, while the latter involves complex collection fiber optics. Further, neither patent combines the two light sources in such a manner as to provide a uniform light source for the display.
Thus there is a need in the art for improved light sources for head-up displays which can provide both adequate brightness of the display and a uniform light source for the display in order to permit clear viewing under high ambient light conditions.